1. Field of the Invention
The present invention relates to a method and apparatus for fabricating a high-frequency circuit substrate of which smaller dielectric losses are desired. In particular, the invention relates to a method of creating a circuit intended for higher frequency bands such as microwave bands and millimeter wave bands, and an apparatus using the same.
2. Description of Prior Art
It is desirable for substrates to be used in microwave, millimeter-wave, and other high-frequency circuits to be made of material having low-dielectric-loss properties so as to suppress dielectric losses in the circuits resulting from the substrates themselves.
FIG. 1 is an explanatory diagram showing that the inherent dielectric loss of a substrate""s own affects a signal flowing on a transmission line. FIG. 1 is a cross-sectional view of the circuit substrate as cut perpendicular to the signal flowing on the transmission line. The reference numeral 101 represents the substrate, 102 the transmission line, 103 a ground electrode, and 104 electric lines of force occurring when an electric signal flows on the transmission line 102. As shown in FIG. 1, the passage of an electric signal through the transmission line 102 makes the electric lines of force 104 run inside the substrate 101. Here, the electric lines of force 104 undergo the influence of the dielectric loss (given by the value of dielectric loss tangent) inherent to the substrate 101. A loss on the transmission line is given by:
loss=factorxc3x97frequency of circuit in questionxc3x97(permittivity of substrate)1/2xc3x97dielectric loss of substrate (dielectric loss tangent). 
The loss caused here converts into thermal energy, causing a phenomenon of heating the substrate.
Fabricating a high-frequency circuit involves the phenomenon described in FIG. 1. Therefore, a substrate having properties of lower permittivity and lower dielectric loss is selected for use. Substrates made of typical organic materials, at low frequencies, show the properties of lower permittivity and lower dielectric loss. In microwave and millimeter-wave bands of 1 GHz or higher, however, the substrates significantly deteriorate in permittivity due to the materials"" potential polarization and frequency response, and thus are not often selected as substrates for use in high frequencies (approximately 1 GHz or higher). For high frequencies, it is common to select inorganic materials such as alumina (permittivity: approximately 9, dielectric loss tangent: approximately 0.001), zirconia (permittivity: approximately 8, dielectric loss tangent: approximately 0.001), and aluminum nitride (permittivity: approximately 8, dielectric loss tangent: approximately 0.001).
Glass such as quartz is low in permittivity (permittivity: approximately 4) and in dielectric loss (dielectric loss tangent: 0.001 or lower) as compared to the inorganic materials. Therefore, it seems to be a promising material for high-frequency substrates for microwave and millimeter-wave bands. However, it is difficult to apply partial machining required of circuit substrates such as xe2x80x9cthrough hole formationxe2x80x9d to glass. Accordingly, it has been seldom used for high-frequency circuit substrates heretofore.
When glass is selected as the substrate material, ultrasonic machining appears to be the effective means for forming through holes in the glass substrate. The reason why chemical processing methods such as etching are not used for glass machining is that glass is a stable material. That is, although glass can be etched by solutions of hydrofluoric acids, phosphoric acids, alkalis, or the like, etching rates are extremely low (nearly 1 xcexcm/h or so). As for sandblasting, sandblasting is capable of in-depth machining only twice or so the thickness of a mask. For example, in the case of forming 100-xcexcm-diameter through holes, the holes cannot be formed beyond 200 xcexcm or so in depth, relative to the mask pattern having 100-xcexcm openings. Here, no through hole can be made if the glass substrate is thicker than 200 xcexcm.
When ultrasonic machining is used to machine a glass substrate, a 100-xcexcm hole in a 500-xcexcm-thick substrate can be made at a machining rate of 1 sec or faster. Besides, the shape of the tool (horn) used in the ultrasonic machining can be devised to make a plurality of holes at a time. Nevertheless, the ultrasonic machining wears the tool in operation, which requires a replacement with a new tool after several times of machining to glass substrates. In addition, there are limitations on the dimensions of the tools. Therefore, the ultrasonic machining is a method hard to apply to mass production processes of large area glass substrates.
Meanwhile, laser beam machining has been already applied to mass production processes including the formation of through holes in alumina substrates and the like intended for high-frequency circuits, without any limitations on substrate sizes. The laser beam machining is, thus, suitable for ordinary substrate machining, whereas its application to glass substrates gives rise to the following problems. A YAG laser, a typical solid-state laser, has a laser wavelength (1.06 xcexcm) which is transparent to glass. Therefore, the YAG laser is hard to apply to glass machining. Concerning excimer laser machining, the present inventors made machining experiments on quartz glass of 500 xcexcm in thickness by using a KrF excimer laser (wavelength: 0.248 xcexcm), and obtained the following results. That is, through holes of the order of 100 xcexcm in diameter could be made at an energy density of approximately 25 J/cm2. Nevertheless, the process conditions had an extremely narrow range such that any smaller energy densities preclude the machining while any greater energy densities create large cracks in glass substrates. This means that the excimer laser machining is an inappropriate method for machining a glass substrate, in terms of application to mass production processes.
It is conceivable that the use of an F2 excimer laser having a wavelength (0.157 xcexcm) shorter than that of the KrF excimer laser could somewhat relax the narrow range of the process conditions for glass substrates. F2 gas is, however, poisonous to humans and therefore the use of the F2 excimer laser in mass production processes is unrealistic.
Now, turning to the case of machining a glass substrate by using an ultrashort-pulse laser so-called femtosecond laser which has a pulse width of no greater than 10xe2x88x9213 seconds. As described in e.g. xe2x80x9cInteraction Between Light and Glass by Ultrashort Pulse Laserxe2x80x94Growing Frequency Conversion Crystal in Glassxe2x80x94xe2x80x9d (pp. 67-73), MATERIALS INTEGRATION vol. 13, no. 3 (2000), the machining to the glass substrate is possible. However, due to high prices and high running costs of ultrashort-pulse laser systems, the application to mass production processes is difficult.
A machining method using a CO2 laser that is used for forming through holes and the like in an alumina substrate can be adopted to execute the perforation of glass substrates under process conditions wider than for the excimer lasers. In addition, since CO2 laser systems are lower in price and in running costs than the other systems, it can be said that the CO2 laser machining is a glass substrate machining method suited to mass production.
Nevertheless, machining glass substrates by using the CO2 laser produces the problem to be described below.
FIG. 2 is a diagram showing the problem that arises when a glass substrate is machined by using a CO2 laser of variable pulse width. In FIG. 2, the reference numeral 201 represents the glass substrate, and 202 a through hole formed by the laser. Moreover, 203 represents upheaval that occurs upon the formation of the through hole 202, 204 the hole diameter of the glass substrate 201 on the laser-irradiated side (top hole diameter), and 205 the diameter on the side opposite to the laser-irradiated side (bottom hole diameter). As shown in FIG. 2, the through hole 202, being formed in the glass substrate 201 by using the CO2 laser of variable pulse width, has a tapered shape as seen in FIG. 2 with the upheaval 203 on the top rim. The ratio of the bottom hole diameter 205 to the top hole diameter 204 and the amount of the upheaval 203 can be modified by changing the pulse width of the laser to adjust the pulse energy applied to the glass substrate. It is impossible, however, to avoid the tapered shape and the upheaval phenomenon.
The present invention has been achieved in view of the conventional problems described above. It is thus a first object of the present invention to provide a laser-based glass substrate machining method capable of coping with mass production processes.
A second object of the present invention is to use this glass substrate machining method to make a low-permittivity, low-dielectric-loss glass substrate applicable as the substrate of a high-frequency circuit intended for microwave and millimeter-wave bands in particular.
A third object of the present invention is to use the above-mentioned substrate achieved by the glass substrate machining method to enhance performances of a radio terminal apparatus and the like.
The present invention is to achieve the foregoing objects through contrivances both to the glass substrate itself and to the laser beam machining method. Specifically, a glass substrate is provided in which the amount of air bubbles in glass is appropriately controlled to improve the workability of the substrate itself. Then, the glass substrate is irradiated with a pulsed laser over a plurality of times during machining, thereby improving the machining shape to the glass substrate.
To realize the technique as described above, the present invention features that the amount of air bubbles in the glass substrate is controlled to improve the laser workability of the glass substrate itself.
Moreover, the present invention provides a glass substrate machining method for machining a glass substrate by using a laser, comprising the step of controlling the amount of air bubbles in the glass substrate to improve the workability of the glass substrate, in which a thin insulator is formed on the glass surface. In this case, the thin insulator on the glass surface may be glass formed by coating. Alternatively, the insulator may be an organic insulator film. When the insulator is an organic insulator film, the thin organic insulator on the glass surface may be formed by coating. Furthermore, the thin insulator formed on the glass surface may be made into a sheet form by using a laminator.
In addition, the present invention provides a method for machining a glass substrate by using a laser, comprising the step of controlling the amount of air bubbles in the glass substrate to form a vacancy only inside of the glass substrate.
Moreover, the present invention provides a method of forming a metal film on a glass substrate, wherein the amount of air bubbles in a glass substrate is controlled so that the glass substrate, after laser machining, has a large surface area on the machined surface due to bubble traces in glass, thereby performing electroless plating simply.
The present invention is also characterized in that: the amount of air bubbles in a glass substrate is controlled so that the glass substrate, after laser machining, has a large surface area on the machined surface due to bubble traces in glass; and a metal film is formed to improve heat radiation property of the metal-film-formed portion.
The present invention is also characterized in that a CO2 laser is used for laser machining.
Furthermore, the present invention provides a glass substrate machining method for using a CO2 laser of variable pulse width as machining means, the method comprising: a first step of executing a single laser irradiation; and a second step of executing a plurality of laser irradiations.
The present invention also provides the glass substrate machining method described above, wherein the pulse width of the laser in the second step is greater than that in the first step.
Furthermore, the present invention provides a method of fabricating a high-frequency circuit by using the glass substrate machining method having the characteristics described above.
Moreover, the present invention provides a radio terminal apparatus comprising a high-frequency circuit fabricated by using the glass substrate machining method having the characteristics described above.
The present invention also provides a radio base station apparatus comprising a high-frequency circuit fabricated by using the glass substrate machining method having the characteristics described above.
The present invention also provides a radar apparatus comprising a high-frequency circuit fabricated by using the glass substrate machining method having the characteristics described above.
As has been described, according to the present invention, glass substrates which are typically difficult to machine can be easily applied to the fabrication of high-frequency circuits. This allows wide, public supply of high-performance circuits and apparatuses.
The foregoing objects and advantages of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings.